my dealer sprung a code on my 01 tip yesterday, PO421 Warm Up Cat Bank ! efficiency below threshold sporadic need to reflash ecm to factory specs? kind of a vag noob, what does this mean? tia-

 

(16805 41A5 Vorkatalysator Bank1 wirkung zu gering)
That's the Audi official code, it means translated Pre-catalytic Converter Bank1 efficiency below threshold.
Do you run a aftermarket Cat-back exhaust? Have you had any work done on your o2 sensors lately?

It is a bit more complex then I'd like to admit; and the following Information may be applicable to your case (or mine), but this is just a general statement that could be used for any Car of any Brand and not necessary applicable for Audi (Disclaimer)! So Maybe it helps to understand how this darn things works....
My first guess would be to check the o2 sensor. An o2 sensor just measures the partial pressure of the oxygen in the gas it is exposed to, determining how much o2 is in it. The signal is then fed to the ECU. The front O2 sensor (precat) is the one the ECU uses for determining how much fuel to add (efficiency?). The sensor has a very narrow range (as opposed to "wideband" sensors) and is most accurate right around stoich. The ECU can not compute the number for a preset value since it doesn't have that much control, instead it cycles up and down around .5 volts on the sensor (usually they have a 0 to 1 volt range, but not sure for Audi, will ask). The average of these cycles/fluctuations will be around .5 volts under light load.

The o2 sensor is only used in closed loop operation, and not in open loop operation. Closed loop operation is just what it sounds like. There is a closed "loop" between the sensor, the ecu, and its output to the injectors, which is then seen by the sensor, and the loop continues. Its also known as a feedback loop. Open loop is the opposite. The ECU no longer cares what the O2 sensor reports back, so there is no feed back loop. The loop is "open" (like an electrical circuit). It uses mass airflow (flow speed, pressure, and temp (turbo cars), or MAP (manifold absolute pressure) combined with known intake volume etc in the non-turbo cars), and a few other parameters like rpm, to pull a number off of a lookup-table and inject that predetermined amount of fuel. This is why we use an SAFC (SAFC is a fuel computer that adjusts fuel/air ratio by modifying the air-flow meter/MAP sensor signal) or similar to intercept and alter the airflow reading sent to the ECU, to change how much fuel delivery we actually get, thereby changing our air:fuel ratio.

So if your DTC output shows "below threshold" it could mean a couple of things are affecting your system. Usually I just clear the code and continue driving like nothing ever happened......but only to the point until I see more associated DTC's coming up, then it's time to visit your favorite Audi Service Shop.

In my personal, humble opinion here is what I think is wrong; because I run an aftermarket Cat-back exhaust it allows the Cat to stay "cooler" and has a less restrained flow. Therefore the entire value based on the data in the ECU is not 100% applicable anymore. So whatever is stored in the ECU to calculate the o2 measurments goes down the drain and as result the ECU responds with numbers that are "below threshold".

For a more curious reason I save the log files to be able to track down these events (because my car is out of warranty I need to know what's going on).

Hope it helps a little bit....

 

thank you Allroad! what a writeup. I appreciate it. best-

 

BTW...the o2 sensor voltage is between 0 to 1Volt for Audi's....